1. Field of the Invention
The present invention relates to a semiconductor laser device formed by a compound semiconductor material, and, in particular to a semiconductor laser device in which an active layer is formed by an InGaAlP type of material.
2. Description of Background
Recently, an InGaAlP type of material has been notable as the material of a luminescent semiconductor device in which laser beams with the wavelength in a range from 0.5 to 0.6 .mu.m are radiated because the material has the maximum energy band gap among mixed crystals of a III-V families type of compound semiconductor other than nitriding materials.
In particular, a double-heterostructure type of semiconductor laser device in which a substrate is formed by a GaAs type of material and an active layer and cladding layers are formed by an InGaAlP type of material on condition that lattice constants of the layers are matched with that of the substrate can be applied for many practical purposes. The reason is because a visible laser beam with 0.6 .mu.m wavelength is continuously radiated from the active layer at a room temperature in the double-heterostructure type of semiconductor laser device.
The laser beam radiated from the above semiconductor laser device can be focused on a small beam spot because the wavelength of the radiated laser beam is comparably short. Therefore, information can be stored in an optical disk in a high density by utilizing the laser beam radiated from the double-heterostructure type of semiconductor laser device. However, it is required to be stably operated at over 30 mW of the light intensity for practical use in the double-heterostructure type of semiconductor laser device.
That is, there are many causes by which the intensity of the laser beam is restrained in the semiconductor laser device. As a main cause, a kink phenomenon has been well known because the phenomenon influences the characteristics between the electric current applied to the device and the intensity of the laser beam radiated from the device. That is, the linearity of the characteristics deteriorates by the kink phenomenon.
Therefore, when the kink phenomenon is generated because the intensity of the laser beam is increased over a kink level, a lateral mode of the laser beam indicating a light intensity distribution in the active layer is deformed so that the characteristics of the laser beam deteriorate. As a result, the semiconductor laser is difficult to utilize as a light source for an optical disk and the like.
Accordingly, the semiconductor laser device is required to keep the linearity in the characteristics between the electric current and the intensity of the laser beam. In other words, the semiconductor laser device with a high kink level is required to obtain a high intensity of the laser beam.
As one of mechanisms to generate the kink phenomenon, a hole-burning effect has been well known. In detail, the induced emission of the laser beam resulting from the recombination of an electron and a positive hole is strongly generated in high intensity light regions in the active layer so that the concentration of carriers such as the electrons and the positive holes are decreased in the high intensity light regions. Therefore, the density distribution of the carriers is deformed in the active layer so that a gain distribution relating to the ratio of the intensity of the induced emission to the electric current applied to the device is deformed. That is, the lateral mode of the laser beam is deformed.
In general, the density distribution of the carriers becomes difficult to deform as a diffusion length of the carrier is increased. In case of the InGaAlP type of materials, the diffusion length of the positive hole is very short so that the kink level is decreased.
As the other main cause by which the intensity of the laser beam is restrained in the semiconductor laser device, so-called Catastrophic Optical Damage (COD) has been well known. In detail, the laser beam radiated in the active layer is absorbed by the active layer so that a pair of the electron and the positive hole is generated in the active layer. Thereafter, recombination heat is generated in the active layer when the pair of the electron and the positive hole is subjected to a non-radiative recombination so that the active layer is heated. Therefore, the energy band-gap is decreased so that the absorption of the laser beam radiated in the active layer is accelerated. As a result, the crystal of the active layer is melted in the high light intensity region so that the semiconductor laser device is destroyed.
The generation of the COD depends on the light intensity in the active layer. Therefore, in cases where a large amount of light is confined in the active layer because the device has a low gain characteristic or the width of the lateral mode of the laser beam is short, the light intensity achieves a COD level at which the COD is generated in the high light intensity region although the laser beam is radiated at the comparatively low light intensity in other regions. Accordingly, the semiconductor laser device with a high COD level is required to obtain a high intensity of laser beams.
Generally, the light intensity in the active layer is reduced by utilizing a thin active layer to raise the COD level. However, in cases where the active layer is thinned in the semiconductor laser device formed by the InGaAlP type of materials, materials with a comparatively large energy band-gap are difficult to utilize as cladding layers which are arranged to sandwich the active layer and are utilized to confine injected carriers into the active layer. The reason is as follows.
In general, as is well known, the energy band-gaps in the cladding layers are respectively larger than that in the active layer. Therefore, carriers injected in the semiconductor laser device are confined in the active layer because the injected carriers cannot move into the cladding layers by the energy barrier indicating the difference in the energy band-gap between the active layer and the cladding layer.
In cases where the active layer is thinned, a threshold value of the electric current applied to the device is increased according to a quantum theory so that the injected carriers have a high energy. Thereafter, the high energy carriers recombine to radiate the laser beam. In this case, the energy band-gap in the active layer is substantially increased because high energy electrons over a conduction band recombine with high energy positive holes under a valence band. As a result, the energy barrier between the active layer and the cladding layer is decreased so that the injected carriers leak to the cladding layers. In other words, the injected carriers cannot be efficiently confined in the active layer.
More, the energy difference between the conduction band of the active layer and the conduction band of the cladding layer in the InGaAlP type of materials is smaller than that in conventionally utilized GaAlAs type of materials. Therefore, it is difficult to prevent the injected electrons from leaking to a p type cladding layer. That is, the electrons are difficult to efficiently confine in the InGaAlP type of materials. Specifically, when the operating temperature is raised, the light intensity is prominently decreased because the electrons have a large energy. Therefore, the operation is difficult when the light intensity in the active layer is high.
In general, the lattice constant of the InGaAlP type of material can be shifted depending on the composition of a mixed crystal in the material. Therefore, in cases where the InGaAlP type of materials are utilized for the semiconductor laser device, it is conventionally required to reduce the difference between the lattice constant of the substrate and the lattice constant of the active layer arranged on the substrate between a room temperature and a growing temperature of the mixed crystal. The reason is as follows.
The increase of the difference between the lattice constants results in the generation of misfit dislocations or the growth of undesirable lattice defects resulting from the generation of stress, so that characteristics of the material easily deteriorate. Specifically, the deterioration of the material characteristics is remarkable in the semiconductor laser device in which the density of the injected carriers and the light intensity in the active layer is high.
A symbol .DELTA.a/a relating to the difference between the lattice constant of the substrate and the lattice constant of the active layer is defined as a lattice mismatch as follows, EQU .DELTA.a/a =(b.sub.1 -b.sub.0)/b.sub.0
where the symbol b.sub.1 represents the lattice constant of the active layer and the symbol b.sub.0 represents the lattice constant of the substrate. The lattice mismatch .DELTA.a/a is required to be within about 0.2% in the conventional semiconductor laser device.
The influence of the kink phenomenon and the COD are further described in detail in the conventional semiconductor laser device.
FIG. 1 is a schematic cross sectional view of a ridge waveguide type of InGaAlP semiconductor laser device with a conventional structure for controlling the lateral mode of the laser beam radiated from the device.
As shown in FIG. 1, a ridge waveguide type of InGaAlP semiconductor laser device 301 comprises:
n type GaAs substrate 302, n type GaAs buffer layer 303, n type InGaAlP cladding layer 304, in InGaP active layer 305, p type InGaAlP cladding layer 306 with ridge shaped section 306A, p type InGaP cap layer 307, and p type GaAs contact layer 308.
The lattice constants of the layers 303 to 308 approximately agree with that of the substrate 302 because the composition of each layer is adjusted. In other words, the lattice constants of the layers 303 to 308 match with that of the substrate 302.
In the above structure of the ridge waveguide type of InGaAlP semiconductor laser device 301, radiative lights are confined in a ridge waveguide region 309 by the ridge shaped section 306A of the cladding layer 306, the cap layer 307 fabricated over the ridge waveguide region 309, the contact layer 308 functioning to absorb the radiative light. Therefore, the laser beam radiated from the device 301 is controlled in the lateral mode thereof. For example, the operation of the above device 301 is described in detail in a literature "Applied Physics Letters, Vol. 56, No. 18, 1990, page 1718-1719".
In cases where both the composition of the mixed crystal forming the active layer 305 and the composition of the mixed crystal forming the cladding layers 304, 306 are set to satisfy the lattice mismatch .DELTA.a/a within about 0.2%, the kink level is about 40 mW. That is, the lattice mismatch .DELTA.a/a determines the maximum intensity of the laser beam designated by the upper limit of the kink level.
FIG. 2 is a graphic view showing the temperature dependence of characteristics between the electric current (mA) applied to the device and the intensity (mW) of the laser beam in the semiconductor laser device shown in FIG. 1.
As shown in FIG. 2, the operation in the semiconductor laser device can be implemented up to the laser intensity 20 mW because the active layer 305 is thinned, for example, to 0.04 .mu.m (400 .ANG.) thick. Moreover, The COD level is a high value such as 51 mW (not shown).
However, the light intensity is remarkably decreased at high temperatures above 40.degree. C. so that the laser intensity is about 10 mW in practice at an operating temperature of about 50.degree. C. Accordingly, the COD level at the operating temperature limits the practical light intensity.
On the other hand, a laser beam with a short wavelength is superior to store information in an optical disk in a high density. The reason is as follows. In cases where the laser beam is focused to make a minute spot by utilizing an optical system, the theoretical limitation of the diameter of the minute spot is proportional to the wavelength of the laser beam.
Recently, a literature "Electronics letters 3rd, Aug. 1989, Vol. 25, No. 14, pages 905-907" has been reported. As described in the literature, an InGaAlP type of mixed crystal is fabricated on a substrate by utilizing an epitaxial growth technique. A crystal forming the substrate has a specific plane direction which leans toward a suitable direction equivalent to a &lt;011&gt; direction from a (100) plane. In this case, a natural superlattice structure regularly directed in a &lt;111&gt; direction is difficult to fabricate in the mixed crystal on the substrate. Moreover, the energy band-gap in the mixed crystal with no natural superlattice structure is larger than that in the mixed crystal with the natural superlattice structure.
Therefore, the wavelength of the laser beam can be shortened by utilizing the mixed crystal with no natural superlattice structure although the composition of the mixed crystal is not changed. However, there is a drawback that the deterioration of the temperature characteristics shown in FIG. 2 is not still solved.
Moreover, as shown in FIG. 1, in cases where the mixed crystal with no natural superlattice structure is utilized in the conventional ridge waveguide type of InGaAlP semiconductor laser device 301, the ridge shaped section 306A of the p type cladding layer 306 becomes asymmetric because the section 306A have a (111) plane direction and the substrate 302 is leaned from the (100) plane.
Therefore, the lateral mode of the laser beam radiated from the laser device with the asymmetric ridge shaped section is unstable as compared with that of the laser beam radiated from the laser device with the symmetric section 306A. As a result, the kink phenomenon can be easily generated although the laser intensity is low.